1. Field of the Invention
The present invention relates to apparatus and methods for treating injuries to bones in mammals, especially injuries to weight-bearing bones. Specifically, this invention relates to external fixation devices for immobilizing a limb or portion of a limb while allowing use of the limb as a weight bearing limb.
2. Description of the Related Art
Injuries to the limbs of mammals, including animals and man, are generally treated by immobilizing the limb until the injury heals. One method commonly used in man and certain other mammals is to immobilize the limb in a hard cast, generally formed of plaster or fiberglass. While the use of such a cast is convenient and effective, casts have disadvantages as well.
First, an injured limb is most often swollen after a break occurs. Placing the limb in a cast will create undue pressure on the swollen limb. Further, as the swelling abates, the cast can become loose, necessitating its removal and reapplication. Further, if the injured limb has superficial lacerations or other injuries, a cast will effectively prevent proper treatment. Plaster or fiberglass casts are also bulky and cumbersome, as well as uncomfortable. Finally, such casts can support only a limited amount of weight. While a man may use such a cast effectively, the cast will be unable to support the weight of larger animals. This is especially true where the animal is not intelligent and will continue to try to use the injured limb as normal.
Attempts to overcome these disadvantages have been made. For example, Walker, U.S. Pat. No. 3,470,873, discloses an adjustable splint which is said to be especially useful to immobilize the injured leg of a bovine animal. The Walker splint includes a pivoting plate to which the foot of the animal is attached, and the animal can walk about with the splint in place. Dawson, U.S. Pat. No. 4,029,090, describes a heavy, hinged framework which can be employed to encase the injured foreleg of a horse. Application of the brace allows the horse to stand but causes rub sores. A simple brace for an equine animal's injured leg, including a support for the hoof, is disclosed in U.S. Pat. No. 4,320,722 to Glassman, et al. Like a plaster cast, these devices are designed to support the entire weight of the animal, through the device, on soft tissues. Especially in the case of a large animal, this is impractical and undesirable. The soft tissue is subject to abrasion from the device, causing open sores to develop, as well as bruising. Pressure on the soft tissues from the device can also lead to the formation of decubitus ulcers which are difficult to treat and, if left untreated can eventually lead to bone loss in affected areas.
External fixation devices which transfer stress directly to bone have also been developed to try to overcome the disadvantages of the prior devices. In U.S. Pat. No. 3,877,424 to Murray, pins are inserted through bone fragments and extend beyond the surrounding soft tissue, where they are bonded to a rigid bridge. However, the Murray fixator makes no provision to allow full weight bearing on the injured limb. Thus it is not useful where the patient is not intelligent and will continue to use the limb, as in bovine, equine, and other veterinary applications.
Finally, U.S. Pat. No. 4,604,996 to Nunamaker, et al., which is incorporated herein by reference, discloses a device for transferring the full weight bearing force of an animal directly to an external fixation device. The device uses pins which pass through the bone proximal the injured bone in the injured limb of the animal. These pins are then secured to a superstructure which transfers the weight of the animal from the pins to the ground. This device retains several shortcomings.
First, the stress placed on the pins in the Nunamaker device increases as the distance from the bone to the superstructure increases. Since the superstructure cannot be placed directly against the bone of the patient, the stress on the pins is very high. In Nunamaker, this is overcome by providing larger pins which are more likely to withstand the stress placed upon them and by providing more pins. Both of these features leads to further problems. Larger pins require that larger holes be made in the intact bone. Larger holes act as larger stress concentrators, leading to a greater chance of bone fracture. Additional pins complicate this. Thus, it is one object of this invention to provide apparatus and a method of using it which are effective for treating an injured mammalian limb, while avoiding the problems recited above. It is another objective that the treated limb be capable of supporting the mammal's full weight immediately after application of the apparatus without reliance on soft tissue support.